Modification of polyamides



Patented July 5, 1949 MODIFICATION OF POLYAMIDES William way Watkins,Buffalo, N. Y., assignor to E. I. du Pont de Nemours & Company,Wilmington, Del., a corporation of Delaware No Drawing. ApplicationJanuary 23, 1945, Serial No. 574,197

12 Claims. (01. 260-72) This invention relates to a process for thetreatment of N-alkoxymethyl polyamides and of N-alkylthio'methylpolyamides, to improve the properties thereof. More'particularly, itrelates to a new and improved process for treatment of N-alkoxymethylpolyamides to render the same more resistant to the action of water andcertain organic solvents and also to raise the softening or meltingpoint temperature of these polyamides.

This application is a continuation-in-part of my copending applicationSerial No. 520.105, filed January 28, 1944, now abandoned.

The copending patent application of Theodore L. Cairns, Serial No.539,195, filed June 7, 1944, now Patent No. 2,430,860, describes andclaims a new class of synthetic linear polyamides of the type generallydisclosed in Carothers U. S. Patents Nos. 2,071,250, 2,071,253 and2,130,948. The polyamides of this kind, generally speaking, comprise thereaction product of linear polymerforming compositions containingamide-forming groups, for example, reacting material consistingessentially of bifunctional molecules each containing two reactivegroups which are complementary to reactive groups in other molecules andwhich include complementary amide-forming groups. These polyamides canbe obtained by the methods given in the above mentioned patents and byother methods, for example by self-polymerization of amonoaminomonocarboxylic acid, by reacting a diamine with a dibasiccarboxylic acid in substantially equimolecular amounts, or by reacting amonoaminomonohydric alcohol with a dibasic .carboxylic acid insubstantially equimolecular amounts, it being understood that referenceherein to the amino acids, diamines, dibasic carboxylic acids, and aminoalcohols is intended to include the equivalent amideforming derivativesof these reactants. The preferred polyamides obtained from thesereactants have an intrinsic viscosity of at least 0.4 and a unit lengthof at least 7, where "unit length is defined as in U; S. Patents Nos.2,071,253 and 2,130,948. The average number of carbon atoms separatingthe amide groups in these polyamides is at least two.

These linear polyamides include also polymers, as for instance thepolyester-amides, obtained by admixture of other linear polymer-formingreactants, as for instance glycol-dibasic acid mix- 2 tures of hydroxyacids, with the mentioned polyamide-forming reactants.

Both the simple and modified linear polyamides contain the recurringamide, groups in which X is oxygen or sulfur and R is hydro gen or amonovalen-t hydrocarbon radical, as an integral part of the main chainof atoms in the polymer. 0n hydrolysis with hydrochloric acid, the aminoacid polymers yield the amino acid hydrochloride, and thediamine-dibasic acid polymers yield the diamine hydrochloride and thedibasic carboxylic acid, and the amino alcohol-dibasic acid polymersyield the amino alcohol hydrochloride and the dibasic carboxylic acid.

The polyamides described above are useful in many ways, particularly inthe form of fibers. However, most of the prior polyamides are insolublein common organic solvents and cannot therefore be used conveniently forthe preparation of solvent cast films and coatings. Certainalcohol-soluble polyamides have been prepared previously, but these areall derivedfrom complicated multi-ingredient systems, or by usingspecial and expensive diamines, such as triglycoldiamine.

The polyamides treated according to the invention of Cairns must containa hydrogen-bearing amide group. This does not mean that all the amidenitrogen atoms in the polyamide must have a hydrogen atom attachedthereto. A polyamide containing both hydrogen-bearing andnon-hydrogen-bearing amide nitrogens, such as can be obtainedby reactinga dibasic acid with a mixture of diprimary and disecondary diamines, canbe used in the process of this invention.

The preferred polyamides treated in accordance with Cairns invention arethe polycarbonamides which include the polyureas, e. g.polydecamethylene urea. Another type of polyamide of special utility isthat prepared by reacting monoaminomonohydrlc alcohols. with dibasiccarboxbylic acids, specifically that prepared by reacting sebacic acidand monoethanolamine. Other examples of particularly useful polyamidesare described in U. S. Patents Nos. 2,071,253 and 2,130,948. However,polysulfonamides (U. S. Patents Nos. 2,321,890 and 2,321,891), e. g. thepolysuifonamide derived from decamethylene-diamine andm-benzenedisulfonyl chloride, can also be used. Additional polymerswhich can be used are the polyurethanes and polythiourethanes (U. S.Patent No. 2,284,637), e. g. the polymer derived fromdecamethylenediisocyanate and decamethylene glycol; polythionamides (U.8. Patent No, 2,201,172), e. g. polydecamethylene thiourea; andpolyhydrazides, e. g. that derived from hydrazine and sebacic acid.

The new products described and claimed in said application of CairnsSerial No. 539,195 include polyamides in which a portion of the nitrogenatoms of the polyamides have substituent alkoxymethyl groups. Saidnitrogen substituted polyamides are prepared by reacting a linearpolyamide which has an intrinsic viscosity of at least 0.4 and containshydrogen-bearing amide groups as an integral part of the main polymerchain, with formaldehyde and a formaldehydereactive organic compoundhaving hydrogen attached to an element of groups V and VI of series 2and 3 of the periodic table, particularly an alcohol or mercaptan, inthe presence of a catalyst consisting essentially of oxygen-containingacid having an ionization constant at least as great as 9.6 x and anequivalent conductance, measured at C. in 0.01 N concentration, nogreater than 370 ohms, cm

The initial polyamides treated by the Cairns invention should be ofsufiiciently high molecular weight to have an intrinsic viscosity of atleast 0.4 (defined as in U. S. Patent No. 2,130,948) to insure theformation of tough products. This means that the initial polyamides arethose prepared from polymer-forming reactants containing complementarypolymer-forming'groups in substantially equimolecular amounts. Thus, ifa polyamide of the diamine-dibasic carboxylic acid type is used, thediamine and dibasic acid used will be in substantially equimolecularproportion. The use of one of the reactants, e. g. the dibasic acid oran amide-forming derivative thereof such as the diester or diamide, inappreciable excess (above about 5%) leads to the formation of apolyamide of too low molecular weight to come within the scope of theinitial polyamides used in the present process.

By the procedures described in said application of Cairns Serial No.539,195, the functional group I'M; d=x

I of the polyamide (in which X is oxygen or sulfur) is converted to thefunctional group sulfur, i. e., a chalcogen of atomic weight less than33, R represents the organic radical obtained by removal of HA from acompound of the formula HAR, wherein said compound represents asubstance of the class consisting of alcohols and mercaptans in whichthe thiol group is attached to an aliphatic hydrocarbon, and said groupsconstitute at least 10% of the amide groups in the main polymer chain.The products treated in accordance with the present invention referspecifically to the substituted polyamides described above and disclosedin greater detail in said Cairns application, of which theN-alkoxymethyl polyamide type is preferred and will be most particularlyreferred to throughout this specification since it represents the typeof polymer described by Cairns with which the present applicant is mostconcerned. The N-alkoxymethyl polyamides prepared in accordance with theprocess of Cairns will unavoidably contain some methylol substituentgroups as well as alkoxymethyl substituent groups.

The N-alkoxymethyl polyamides as produced by the above stated reactioncan be readily dissolved in numerous commonly used organic solvents, forexample methanol, ethanol, waterethanol mixtures, and acetone-methanolmixtures. Structures formed from such solutions have exceedinglyinteresting properties. For example, many of the resultant structureshave a high elastic recovery from stretch to from elongations of 100% ormore), an excellent pliability and a good resistance to repeatedflexings, even at low temperatures. Unless these polyamides are heatedat a relatively high temperature for an extended period of time, asdescribed in the Cairns application, they are extremely sensitive towater, especially boiling water or steam, in that they will be greatlyweakened thereby. They are, furthermore, soluble in many common organicsolvents and have relatively low softening and melting points. Cairnsdiscloses in his application that these deficiencies can be overcome bysubjecting the products to a baking treatment. Such a baking treatmentmust, however, be carried out at a relatively high temperature and foran extended period of time which is not only expensive and inconvenient,but often results in an objectionable degradation of the product. Thecopending application of Cairns Serial No. 507,745, filed October 26,1943, now abandoned, discloses that where the methylol groups areremoved from the N-alkoxymethyl polyamides, the latter are even morediiiicultly insolubilized by the mere application of heat.

It is an object of this invention to provide a new and improved methodfor treatment of a polyamide containing the functional group IiHCHzAR)to render it less easily weakened in boiling water, to increase itsmelting point and to reduce the solubility thereof in organic solvents.

It is another object of this invention to provide a method for thetreatment of an N -alkoxymethyl polyamide to render it less extensivelyweakened in boiling water, to increase its melting point, and to reducethe solubility thereof in organic solvents without objectionablydegrading the same.

Other objects of the invention will appear hereinafter.

The objects of the invention may be accomplished by incorporating anacid with an N-alkoxymethyl polyamide. Preferably, the acid ismaintained in contact with the polyamide until the zero strengthtemperature of the polyamide is increased at least 5 C.

Since the N-alkoxymethyl polyamides which are to be treated with an acidin accordance with the present invention gradually soften by theapplication of heat, it is difficult to determine the polyamide has azero strength is made as follows:

-An electrically heated, smooth, chromium plated cylinder /2 inch indiameter) is equipped with a surface pyrometer to measure the surfacetemperature of the cylinder. The electrical heating element in thecylinder is attached to a source of electric current which will heat thecylinder at a rate of 70 C. per minute between 100 C. and 170 C. Thestrip of synthetic linear polyamide to be tested for zero strengthtemperature is positioned to firmly contact the cylinder closelyadjacent the 'pyrometer and a small weight (12 pounds per square inchcross-section of the film) is attached to the bottom end of the film. Inorder to insure firm contact between the film and cylinder, the film ismade to contact the cylinder over an arc of about 45. In carrying outthe tests, it is most convenient to use a film having a thickness of0.002 inch, 9, width of 0.116 inch and a length of 3 to 4 inches, and aweight of 1.35 grams is attached to the bottom end thereof. The film islowered into contact with the cylinder when the latter has. atemperature of 100 C, The temperature at which the film breaks isobserved by a pyrometer reading. This temperature is taken as the zerostrength of the film. Throughout the specification and claims referenceto zero strength temperature designates a breaking temperature asdetermined in the above-defined manner.

The following detailed examples are given to illustrate certainpreferred processes for practicing the present invention, it beingunderstood that the details set forth in the examples are for thepurpose of illustration and are therefore not to be taken as limitingthe scope of the invention.

Example I (This example illustrates a preferred procedure for convertinga highly substituted, N-methoxymethylpolyhexamethylene adipamide to amore desirable product having the same high degree of elasticity, butbeing resistant to hot water.)

A solution of N methoxymethylpolyhexamethylene adipamide (about 52%amide substitution) was prepared by stirring a mixture of 48.0 parts ofthe polymer, 0.48 part of maleic acid, 89.6 parts of methanol and 22.4parts of water for one hour at 60 C. A film was prepared by flowing theviscous solution on to a glass plate;

After evaporation of the solvent at room temperature, the film (still onthe plate) was heated in an oven for six hours at 60 0. Analysis showedthat this heat treatment removed 27% of the methoxyl groups from theoriginal methoxymethyl polyamide. The resulting, highly elastic, rubberyfilm was unafiected by boiling water or steam, was infusible at 300 C.and was insoluble in hot aqueous alcohol. In contrast, a film preparedfrom a similar solution containing no maleic acid and similarly heated,disintegrated in boiling water, had a zero strength temperature of 111C. and was alcohol soluble. In addition, the film prepared in thepresence of maleic acid exhibited higher tensile strength (wet or dry)than the film prepared from a solution containing no acid and wasgreatly superior in resistance to the penetration of gasoline (0.005 vs.0.031 can/sq. ft./24 hrs). Both the 8 films showed over elastic recoveryfrom 300% to 500% elongation, were very pliable and had a. high degreeof resistance to repeated flexings and did not crack when bent sharplyat temperatures as low -55 0.

Example I! A solution of N-methoxymethyl-polyhexamethylene adipamide(49% of amide groups substituted with methoxymethyl groups) was preparedby stirring a mixture of parts of the polymer, 120 parts of methanol, 30parts of water and 1 part of maleic acid for one hour at 60 C. A filmwas prepared by flowing the viscous solution on to a glass plate. Afterevaporation of the solvent at room temperature, the film was removedfrom the plate and suspended in an oven for five minutes at 100 C. Theresulting,

No Maleic Maleic Acid Acid Tefiisillie )strength in lb./sq. in. (50%1,710 550. Tenslle s trength in lb./sq. in. (wet)-- 0. Break Elongation,per cent 395 376. Elastic recovery, per cent at 100% 06 93.

stretch.

Pliability over 200 over 200. Flexes required to effect cracking over33,000,000. 295,000

1 Schiltknecht flex test at 25 0. Bull. Alfr d S i; C W Fifth Avenue,New e u or o 20 Yorlr city).

Thus, the maleic acid catalyst conferred infusibility, insolubility, hotwater resistance, flex durability and higher tenacity (wet and dry)without changing other desirable properties, such as high elongation,elasticity and pliability.

Example III (This example illustrates a preferred acid treatment of amoderately substituted N- methoxymethyl-polyhexamethylene adipamide inorder to improve its resistance to hot water, to make it less permeableto hydrocarbons such as gasoline and to improve its resistance tocracking on repeated flexing.)

Films were prepared and heat treated as described in Example I from lesshighly modified N-methoxymethyl-polyhexamethylene adipamide (about 33%amide substitution). The film pre pared from a solution containingmaleio acid was resistant to boiling water and showed great resistanceto cracking on repeated flexing, as compared to the water sensitive filmprepared from a solution containing no maleic acid. It had a: zerostrength temperature of more than 300 C. It had an elastic recovery of74%. Analysis showed that 16.6% of the original methoxyl groups had beenremoved.

Films prepared by coating the maleic acidcontaining solution onsynthetic or natural rubber sheets and then heating at 100% C. for a fewminutes showed excellent adhesion to the rubber substrate and were notdamaged by exposure to steam under rubber vulcanization conditions.

Example IV (This example illustrates the use or a weak dibasic acidcatalyst which produces a product having extremely good adhesion tosurfaces such as glass.)

A solution consisting of 48 parts of N-methoxymethyl-polyhexamethyleneadipamide (about 33% amide substitution), 89.6 parts of methanol, 22.4parts of water and 4.8 parts of sebacic acid was fiowed on a plate.After air drying, the film while still on the plate, was heated in anoven at 100 C. for twenty-four hours. The resulting film was then soadherent to the glass plate that it could not be stripped oil even'after immersion in water for seven days. The film had a zero strengthtemperature of more than 300 C.

Example V (This example illustrates the use oi a strong acid catalyst inorder to accelerate the speed and degree of modification.)

A solution was prepared as described in Example III using 0.48 part ofp-toluene sulfonic acid instead of the maleic acid. A film prepared fromthis solution and air dried for about fortyeight hours was unafiected byboiling water and was strong, pliable and elastic. Analysis showed thatthis treatment removed 49% of the methoxyl groups in the originalpolymer. Interesting data on this film are:

Catalyst I No Catalyst Tensile strength 4,7(XJ lb./sq. in 4,6(1) ib./sq.in. Elongation, per cent 328% 387%. Piiability 48. Elastic Recovery, percent 62% 71%.

at 100% stretch.

Eiiect of boiling water..... None Mel Zero strength temperature.infusible at 300 C... 107 C A similar film heated for six hours at 60 C:on a glass plate showed increased tensile strength and elasticity withmuch greater stiffness (lower pliability) and lower extensibility.Analysis revealed that in this case the heat treatment brought about an88% loss of methoxyl groups;

Interesting data on this film are:

Catalyst No Catalyst Tensile strength 7,310 lb./sq. in 4,1001b./sq. in.Elongation, per cent 49% 441%. Pliability 11 48. Elastic Recovery from84% 70%.

break elongation.

Eflcct of boiling water... None Melts. Zero strengthtemperatureiniusible at 300 0... 107 0.

It will be seen that a strong acid catalyst such as p-toluenesulfonicacid confers hot water resistance without any heat treatment and givesstiffness with extended heat treatment.

Example VI found that this treatment increased the tensile strength:both wet and dry, increased the stillness (i. e. decreased thepilability) and lowered the elongation. The film also had excellentresistance to boiling water. The zero strength temperature of the filmwas increased more than Example VII (This example illustrates themodification of N isobutoxymethyl polyhexamethylene adipamide with anacid catalyst.)

Asolution was prepared by stirring 48.0 parts ofN-isobutoxymethyl-polyhexamethylene adipamide (about 58% amidesubstitution), 89.6 parts of methanol, 22.4 parts of water and 0.48 partof maleic acid at 60 C. for one hour. Films were prepared by flowing thesolution on glass plates and air drying. A film baked at 100 C. for onehour (on the plate) was resistant to boiling water, was infusibie at 300C. as against a zero strength temperature of 194 C. for a similaruncatalyzed film, and had good tensile strength, elongation, pliabilityand an elastic recovery of 72% at an elongation of 100%.

Example VIII (This example illustrates the use of a monobasic acidcatalyst to convert one of the preferred N methoxymethylpolyhexamethylene adipamide products (32.9% amide substitution) to a hotwater resistant product.)

A film prepared as described in Example 111 containing 0.48 part offormic acid instead of the maleic acid, was heated at 100 C. for sixhours on a glass plate. This brought about a loss of about 4% of theoriginal methoxyl groups. The resulting film was less affected byboiling water than control film" heated under the same cone ditions, butcontaining no acid catalayst. The zero strength temperature of the filmwas increased over 20'' C. The treated product had essentially the samepliability as the original polyamide and had an elastic recovery of 83%when stretched 100%.

Example IX A solution of N-methoxymethyl derivative of an interpolymeror hexamethylene diammonium adipate and hexamethylene diammoniurn sebacate (30:70) (37% amide substitution) was prepared by stirring a mixtureof 35 parts of polymer, parts of ethyl alcohol and 1.75 parts of citricacid at C. for one hour. A film was prepared by flowing the warmsolution on a glass plate and allowing the solvent to evaporate at roomtemperature. The dried film was suspended in an oven at C. At the end often minutes, the film was found to be infusible although it stillpossessed excellent elasticity. These properties were substantiallyunchanged after two hours in the oven. Analysis showed that 10% of themethoxyl groups had been removed. A similar uncatalyzed film was foundto have a zero strength temperature of C.

Example X (This example describes the use or an N-alkoxymethyl polyamideas a coating for various synthetic films.)

A solution of the N-methoxymethyl derivative of an interpolymer ofhexamethylene diammonium adipate and hexamethylene diammonium sebacate(30:70) (42% amide substitution) was prepared as in Example IX, usingmaleic acid as the catalyst in place of citric acid.

9 A thin coating was obtained by flowing this solution on. a film ofpolyvinyl alcohol and evaporating the solvent at 40 C. At this stage,the coating could be stripped from the base sheet and was found to havea zero strength temperature of about-100 C. When, however, the coatedfilm was heated at 100 C. for five minutes, the coatme could not beremoved and was found to be infusible. Analysis showed that 3% of themethoxyl groups had been removed. I

The solution described above was coated on .both sides of polyvinylalcohol, regenerated cellulose and cellulose acetate films. The coatedfilms were then subjected to a temperature of 100 C. for a period offive minutes. At the end of this period, it was found to be impossibleto remove the coatings even on prolonged soaking in hot water. Asubsequent soaking in cold water for one month failed to remove thecoatings.

The process of the present invention is applicable to the treatment ofany N alkoxymethyl polyamide regardless of the specific alkoxy group,the specific polyamide or the number of alkoxymethyl substituent groupscontained therein.

From the viewpoint of the desirability of the resultant product, thepresent invention will have quent formation of the product from thesolution,

for example yarns, films, fabrics, sponges, coatlugs and the like. Theinvention however also contemplates the incorporation of the acidcatlyst in the preformed structure. When the acid catalyst isincorporated in a solution of the polyamide, alcohols which may bepresent in the solvent medium are removed by evaporation, preferably ator about room temperature, before the elevated temperature is applied toeffect crosslinking, as is shown in the examples. Removal of thealcohols, such as methanol for example, from any solvent mediumcontaining the same, prior to or during the heat treatment, makes itpossible to split off the methanol contained in the polymer moleculewith concomitant cross-linking,

The present invention is applied in particular to N-alkoxymethylpolyamides and the other related compounds described herein which have.after their formation, been freed from the components of the reactionmixture.

The invention may be carried out with any acid, organic or inorganic,and preferably by heating at a temperature above C. As indicated by theexamples, acids of any type and strength can be used. Preferablyhowever, acids of medium or high strength are used since the time of thetreatment can thereby be greatly shortened. The'preferred'acids have anionization constant for the first hydrogen of between 1 x 10- and 1 x10--*. It is furthermore preferred that the acids bereasonably'non-volatile under the heating conditions used.

The time and temperature under which the treatment is carried out dependsomewhat upon the type of polyamide and the catalyst used. A

range of 0 C. to 300 C. or more, and preferably is relatively long.These times were chosen for simplicity of experiment; substantiallysimilar results will be obtained by considerably shortening thetreatment time of the examples. As is shown in Examples IX and X,relatively short times can be used successfully. Generally, the minimumtime of treatment is preferred to avoid heat degradation of thematerial.

One skilled in the art will .understand, for example, that the strengthof the acid in part determines the time of treatment to obtain aparticular result at a given temperature and that the stronger the acidthe shorter will be the time of treatment. The stronger acids fallingwithin the scope of the invention, for example ptoluenesulfonic acid,are effective at room temperatures. Acids of intermediate strength, suchas maleic acid, are more useful at temperatures in the range of 60 C. to120 C., the time of heating in the case of unsupported films being froma few seconds to an hour. Weak acids such as sebacic acid are preferablyused at temperatures of C. or higher for a period of an hour or more.

Since the acid used in the treatment is not consumed thereby, it must beassumed that the acid acts in a catalytic capacity. Being a catalyticagent, the quantity of acid to be employed is not critical above aminimum. In general, quantities of the order of 0.01% and 20% based onthe polyamide derivative are found to be useful. The quantity to be useddepends upon the particular. polymer, the particular catalyst and thereacting conditions, and the product desired. When it is desired toproduce a soft elastic yarn, it is generally preferred to use about 0.5%of the acid catalyst. On the other hand, when it is desired to produce ahard, adherent film, it is generally desired to have the acid present ina quantity of about 10%.

As has been shown in the examples, the formed alkoxymethyl polyamide maybe treated with the catalytic acid and baked, or in many cases thecatalytic acid may be added to the solution from which the formedstructure is produced. Similarly, it is sometimes satisfactory to leavethe catalytic acid in the structure, but it will usually be foundadvantageous to remove it by thorough Washing.

Analysis of the polyamide shows that the above described acid treatmentwill remove a number of the alkoxy groups from the polyamide. The numberof alkoxy groups removed by the baking treatment may vary over areasonable range and will determine the characteristics of the finalproduct. For example, starting with an N-methoxymethyl-polyhexamethylene adipamide in which 30% to 60% of theamide groups have been converted into N-methoxymethyl amide groups, ithas been found that when an acid treatment is comparatively mild (lessthan 8% of the alkoxy groups removed), the pliability of the product andthe resistance to cracking on repeated flexing is not materially changedwith respect to the original N-methoxymethyl compound. When thetreatment is of moderate intensity (suflicient to remove from 10% to'30% of the alkoxy groups), the flexibility is essentially unchanged, butthe resistance to cracking on repeated flexing is greatly increased.Finally, when the treatment is severe (more than 30% of the alkoxygroups removed). both the flexibility and the resistance to cracking onrepeated flexing are decreased. Using anN-methoxymethyl-polyhexamethylene adipamide as stated, it is preferredthat the treatment be carried out so that between 2% to 30% of thealkoxy groups have been removed. The removal of between 2% and 30% ofthe alkoxy groups will produce an exceptionally desirable elasticproduct. It might be pointed out in this connection that the optimumdegree of removal of alkoxy groups increases with the degree ofalkoxymethyl substitution on the original polyamide and that. of course.the optimum degree will vary depending on the characteristics which itis desired to have in the final product.

Although I do not wish to be limited by any theory involved in themechanism of the invention and the reactions which take place as aresult of the acid treatment, there is definite evidence as a result ofthe many chemical analyses that the acid treatment causes across-linking to take place between the molecules of the polyamide. Thefollowing theory is offered since it appears to be a plausileexplanation of the mechanism of the insolubilization treatment of thepresent invention. Definite proof of this theory is however lacking.

By removing the alkoxy portion of a number of the alkoxymethyl groups.the hydrogen-bearing nitrogen atoms of one or more molecules arebelieved to be cross-linked to the alkoxymethylbearing nitrogen atoms ofother polyamide molecules with the elimination of alcohol, the acidapparently functioning in the capacity of a catalyst. The followingillustrative reaction shows how the nitrogen atoms of the adjacentmethoxymethyl polyamide molecules containing both hydrogen-bearing andmethoxymethyl-bearing nitrogen atoms may be cross-linked in the presenceof an acid catalyst with the elimination of methyl alcohol:

' I l cid I 8 IiT-CHg- O-CH H I cstslystl lf-OH's-II 2CH'OH ing orcoating of the basic polyamide. In addition, the soluble polyamidederivatives have properties which for some purposes were a distinctimprovement over the basic polyamide. Reference is made especially tobetter pliability especially at low temperature and to definitelvsunerior elasticity. It is true that it is known to insolubilize thesoluble polyamides, containing both alkoxy groups and methylol groups,by treatment with heat. This method. however, requires so long anexposure at the elevated temperatures that the product is frequentlydecomposed to an objectionable extent. Additionally, the time is toolong for many practical applications. The acid insolubilization of thisinvention provides a method of securing by practical conditionsinsoluble derivatives of these alkoxymethyl olyamides, whether or notthey contain methylol groups. thus largely increasing the field ofutility of these materials. Quite as important. the insolubilizationtreatment, unless it is continued for too long a-time. does notmaterially alter the valuable characteristics of the solublealkoxymethyl polyamides such as pliability and marked elasticity. Itdoes contribute the very important characteristics of resistance toboiling water and insolubility in common solvents, and the raising ofthe fusing point is well above any temperature encountered in theservice of the article. There is thus available in a practical way a newchemical compound of intensely interesting properties in a very broadapplication.

The products of this invention find utility as yarns, fibers. films.adhesive compositions, coatings, sponges, coatings and interliners forfabrics. safety glass interlayers and similar formed products.

Since it is obvious that many changes and modifications can be made inthe above described details without departing from the nature and spiritof the invention, it is to be understood that the invention is not to belimited to the details described herein except as set forth in theappended claims.

I claim:

I. In the process of treating a linearpoiyai'nide having an intrinsicviscosity of at least 0.4 and containing as an integral part of the mainpolymer chain recurring groups of formula which are separated by anaverage number of carbon atoms of at least 2 and wherein A represents achalcogen of atomic weight less than 33, R represents the organicradical obtained by removal of HA from a compound of formula HAR.wherein said compound represents a substance of the class consisting ofalcohols and mercaptans in which the thiol group is attached to anallphatic hydrocarbon. and X represents a chalcogen of atomic weightless than 33. said groups constituting at least 10% of the amide groupsin the major polymer chain. the steps which comprise incorporating withsaid polyamide a catalytic amount of an acid having an ionizationconstant for the first hydrogen of between 1 x 10- and 10 x 10- andmaintaining said polyamide in contact with said acid until the zerostrength temperature of the polyamide is increased at least 5 C.

2. The process of claim 1 in which the acid is incorporated in analcohol solution of the polyamide and in which the-alcohol issubsequently removed without removal of the acid.

3. The process of claim 1 in which the acid is incorporated in a formedstructure by soaking the formed structure with a solution of the acid.

4. The process of claim 1 in which the acid is present in an amountbetween 0.01% and 20% by weight, based on the weight of the polyamide.

5. In the process of treating a linear polyamide having an intrinsicviscosity of at least 0.4, and containing, as an integral part of themain polymer chain recurring groups of formula r ucmo-nk l) =0 which isseparated by an average number of carbon atoms of at least 2, saidgroups constituting at least 10% of the amide groups of the main polymerchain. the steps which comprise incorporating with said polyamide from0.01% to 20% by weight, based on the weight of the polyamide, of an acidhaving an ionization constant for the first hydrogen of between 1 x 10-and 1 x 10- and maintaining said polyamide in contact with said aciduntil the zero strength temperature of the polyamide is increased atleast 5 C.

6. Theprocess of claim 5 in which the polyamide in contact with the acidis baked at a temperature of from 50 to 150 C., until the zero strengthtemperature of the polymer is increased at least 5 C.

7. The process of claim 5 in which the acid is maleic acid.

8. The process of claim 5 in which the acid is phosphoric acid.

9. The process of claim 5 in which the acid is p-toluene sultonic acid.

10. The process of claim 5 in which the polyamide is N-methoxymethylhexamethylene adipamide polymer. I

11. The process of claim in which between and of the amide groups of thepolyamide are the methoxymethyl substituted amide groups.

12. The process of claim 10 in which the methoxy portions of the methoxymethyl groups are removed from at least 2% and not to exceed 30% of thetotal number of methoxymethyl groups.

- WILLIAM WAY WATKINS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

